Show Addon Panel in Sequencer only, not Image Preview
Set bl_category to the tab name you want to use. For example, bl_category = 'Strip'. You can use a new name too, to create a new tab.

To only show your panel in the sequencer and sequencer+preview modes, and not the preview mode, add this to your panel class:

@classmethod
def poll(cls, context):
    return context.space_data.view_type in {'SEQUENCER', 'SEQUENCER_PREVIEW'}


If you want to have it only in sequencer mode, you can change it to:

@classmethod
def poll(cls, context):
    return context.space_data.view_type == 'SEQUENCER'


PS: I found this bit of code by inspecting the sources of a built-in Blender panel that appears in the sequencer mode and not the preview mode. Just right-click and choose "Edit Source".